Port Forwarding question (simple?)

The background:
I recently set up a little Netgear RP614 router/hub for my home. It's my first experience with any kind of routing. I used to just have my dsl hooked right to my PC with Norton Internet Security doing the firewall duties. I've now uninstalled the Norton because the Netgear seems to do all of the same functions.

Except when it comes to allowing traffic through. With Norton, I basically just said "allow traffic to Apache". Now I gotta deal with forwarding.

My understanding of port forwarding is that when I set the router to forward port 80 to my pc's assigned IP address, it will route ALL traffic on port 80 to my pc. I would like to do that because I have an apache server running on this computer that I use for testing. It would be nice to allow access from outside.

The question:
If I configure the router to forward port 80 to my pc, will that keep other pc's on my little LAN from browsing the web? When a browser requests data from a webserver on port 80, doesn't the traffic come back on port 80 also? So if I've set up forwarding, will the router know the difference and route that traffic to the computer that requested it rather than obeying the forward?

The router uses a four-part key to distinguish connections, remote IP, remote port, local IP, local port. So connects from yoru LAN to other web have a remote port 80 and local port some high number -- on traffic flowing both ways. Traffic from the world to your web box has locla port 80 on traffic in both directions. The router will have no problem distinguishing them.

Is the same true then for other applications? In particular, I'm thinking of Kazaa, which operates on port 1214. I read somewhere that I might need to set up forwarding if I wanted p2p to work right - and if I do, it might keep other machines on the LAN from sharing with the world. Thoughts?